Closes https://github.com/zed-industries/zed/issues/52576
## Overview
Adds prompt caching support for Anthropic Claude models when accessed
via OpenRouter.
This mirrors Zed's native Anthropic provider by using explicit per-block
`cache_control` breakpoints:
- A long-lived 1-hour breakpoint on the system message's last text
block, covering the tools and system prefix.
- A default 5-minute breakpoint on the last `cache: true` conversation
message.
The implementation intentionally avoids OpenRouter's top-level automatic
`cache_control` field so routing remains available across
Anthropic-compatible providers including Anthropic, Bedrock, and Vertex
AI. It also adds `session_id` sticky routing from the request thread ID
and maps OpenRouter cache usage fields into Zed's token usage
accounting.
Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable
Release Notes:
- Fixed Anthropic prompt caching when using OpenRouter.
---------
Co-authored-by: Anant Goel <anant@zed.dev>
